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How to call a function from another script without referencing it?

I'm trying to make a simple script to save and reuse resources instead of instantiating and destroying them all the time. However, I don't understand how do I call these functions without referencing them on the scripts...

I want to do something like iTween, where you only need the script on your folder to be able to call "iTween.MoveTo" and all the other functions. I tried to open and mimick it, but it still gave me errors with my functions, saying that I need a reference to call the functions. How do I do it without referencing, like iTween? I'm using C#